This application claims benefit of Japanese Application No. 2000-249925 filed in Japan on Aug. 21, 2000, the contents of which are incorporated by this reference.
1. Field of the Invention
The present invention relates to a printer device such as a sublimation type thermal transfer printer.
2. Description of the Related Art
Thermal transfer printer devices, with which the display of sharp, full-color images is possible, have become commonplace devices for making a hard copy from a video image stored in a personal computer or captured by a camera-integrated video tape recorder, an electronic still camera, or the like.
In a thermal transfer printer device, the recording paper is sandwiched under pressure along with an ink sheet between a platen roller and a thermal head. The ink sheet consists of a base film coated with a heat-sublimable dye, and is disposed such that the heat-sublimable dye is pressed against the recording paper. A plurality of heating elements are provided on one side of the thermal head, and when current is sent through the thermal head these heating elements generate heat according to the printed data, and heat the heat-sublimable dye via the base film. This results in the heat-sublimable dye being sublimated and transferred onto the recording sheet.
Portable printers must be made more useful by being operable even in places where no AC power supply is available. Specifically, it must be possible to drive a portable printer using only a battery as the power supply. When portability is emphasized, a battery with a relatively small capacity is employed.
Batteries run down as they are used, and the supply voltage gradually drops according to how run-down the battery is. Therefore, even with a portable printer, when an AC power supply is available, it can be used instead of the battery in order to reduce battery consumption and extend the service life of the battery. In view of this, the use of an AC power supply is generally given priority, and when the connector of the AC power supply is connected, the printer automatically switches from battery use to AC power supply use.
Meanwhile, the printing density on a recording paper is determined by the temperature of the heating elements. Specifically, the printing density can be easily varied by varying the amount of current supplied to the heating elements of the thermal head. A method employed for varying the amount of current flowing to the heating elements is to vary how long the current is allowed to flow to the heating elements (hereinafter referred to as the ON time).
This control of the ON time is performed by a CPU incorporated into the device. Since a relatively stable power supply voltage is obtained during the supply of power using an AC power supply, the CPU is able to control current flow without regard to fluctuations in the power supply voltage. In contrast, when a battery is used, the power supply voltage that is supplied varies according to the remaining charge of the battery, so the CPU must monitor the supply voltage and correct the ON time according to the monitoring results.
Japanese Patent No. 3,013,042 discloses a thermal printer device having two power supply inputs (a battery and an AC adapter), in which there is a component for distinguishing between the AC adapter input and battery input, and the internal resistance is set on the basis of a signal from this distinguishing component.
However, frequent monitoring of the supply voltage increases the load of the CPU and also makes printing take longer. Consequently, the supply voltage is generally only monitored when the power is turned on and prior to the start of printing, and the like.
The user, though, may plug in the connector of the AC power supply during printing in which the battery is being used as the power supply. If this happens, the power supply will be switched from the battery to the AC power supply, which may result in a change from the low supply voltage of the discharged battery to the high supply voltage of the AC power supply. Specifically, the current flowing to the heating elements rises, there is a sudden change in the printing density, and print quality suffers.
Conversely, it is also conceivable that the user may unplug the connector of the AC power supply in the midst of printing in which the AC power supply is being used as the power supply. If this happens, the power supply will switch to the battery, the lower supply voltage of the discharged battery will decrease the amount of current flowing to the heating elements, there will be a sudden change in the printing density, and print quality will again suffer.
Good print quality can be maintained if the sequence of monitoring the supply voltage is carried out frequently and the ON time is varied according to the monitoring results, but in this case printing takes longer, as mentioned above.
It is an object of the present invention to provide a printer device with which a decrease in print quality can be prevented, without leading to a longer printing time, by making it possible to prohibit switching from a power supply using the battery to a power supply using an AC power supply in the midst of printing.
The printer device of the present invention comprises a thermal head which prints images on paper on the basis of image data, a power supply switching component which switches a power supply input from a battery power supply and an AC adapter and applies this input to the thermal head, and a controller which, once a print operation with the thermal head has been commenced by switching the power supply switching component so that electrical power supplied from the battery power supply is applied to the thermal head, prohibits the switching of the power supply input by the power supply switching component until the print operation is completed, even if the AC adapter is connected in the midst of the printing of one page.
Also, the printer device of the present invention comprises a thermal head which prints images on paper on the basis of image data, a power supply switching component which switches a power supply input from a battery power supply and an AC adapter and applies this input to the thermal head, and a controller which, once a print operation with the thermal head has been commenced by switching the power supply switching component so that electrical power supplied from the battery power supply is applied to the thermal head, controls the power supply switching component such that the power supply input is switched from the battery power supply to the AC adapter upon completion of the print operation, if the AC adapter is connected in the midst of the printing of one page.
Other features and advantages of the present invention should become clear from the following description.